This invention relates to communication systems employing a communication channel which is shared among plural system users. More particularly, the invention relates to a technique for user equipment seizure of the channel on a reasonably equitable opportunity basis.
As used herein, the term "channel" has reference to both simplex and duplex channels and includes, as well, any of wire, radio, or other signal transmission media.
For convenience of discussion, and without limitation, the invention is herein considered in terms of its application in a mobile radiotelephone communication system. In like manner, the term "mobile unit" which will be used in the consideration, refers to a movable radio transmitting and receiving unit, regardless of whether the unit is carried by vehicular, human, or other means, and whether or not the mobility is in a terrestrial environment.
Various techniques have been used in the art for gaining access to shared communication channels. The R. H. Herrick U.S. Pat. No. 2,616,032 teaches a camp-on busy technique in which a mobile unit simply stays tuned to a desired channel that happens to be busy. The unit user activates a circuit to monitor base station transmitted carrier; and when it goes off the air, the unit transmitter is automatically turned on to seize the channel.
The U.S. Pat. No. 3,513,264 of J. Baer relates to a communication satellite system in which a routing center receives call requests, assigns specific channels to each call, and thereby avoids the problem of seizure attempt collisions in which plural mobile units simultaneously try to gain access to a shared channel.
U.S. Pat. No. 3,377,435 to A. H. Lippert teaches an idle channel marking communication system in which, for example, a predetermined tone is transmitted on the shared channel when it is available for message transmission. Mobile units sharing the channel and detecting the idle marking tone lock to the channel automatically to await either an incoming paging message or an indication by the unit user that it is necessary to initiate an outgoing call. In the latter case, a particular program, or sequence, of tones is transmitted in order to seize the idle channel for the exclusive use of a single unit; and such a program is shown, for example, in FIG. 7 of the patent. If plural units, of approximately equal received signal strength at a system base station, attempt simultaneously to seize the channel, their respective automatic number identification (ANI) sequences cause garbling; and the system control terminal requires all such plural units to retry.
It has been found that it is possible for automatic equipment of multiple subscribers to come into near simultaneous contention for a shared channel with the result that seizure collisions occur recurrently, and none of the contending units actually gains access to the channel. Thus, the channel is, in effect, blocked even though it is not actually in use. The impact of this possibility has been reduced, to a certain extent, by building some measure of randomization into mobile unit automatic channel seizure algorithms as taught, for example, by Z. C. Fluhr and P. T. Porter in "Advanced Mobile Phone Service: Control Architecture," Vol. 58, No. 1, January 1979, Bell System Technical Journal, at P. 55, and in columns 20-23 of the G. F. McClure et al. U.S. Pat. No. 4,028,500. However, some mobile unit users in radiotelephone systems which are designed for randomized seizure algorithms have gained an apparent grade-of-service advantage by employing automatic circuits, sometimes called "instantaneous channel-grabbers," to seize a channel almost as soon as idle marking tone is detected on the channel. An example of such a circuit is shown in the C. J. Sechan U.S. Pat. No. 4,103,106.